Conventionally gold wires are used as bonding metal wires, but in this case the cost becomes higher, and the long period reliability of the junction portion bonded to the aluminum electrode on a silicon chip is disadvantageously low, and accordingly it is thought that wires comprising such as copper, aluminum, palladium, or metal alloys including any of them can be used instead of gold.
Furthermore, it is required to make the top end of the wire with a spherical shape so as not to have any directionality in a wire bonding process. In the conventional gold wire the wire, is melted to have a spherical shape by conducting a discharge between the top end of the wire and a movable electrode (discharge electrode) by a high voltage application therebetween to give heat to the top end of the wire.
In this case, the polarity of the applied voltage is made such that the electrode is the plus and the wire is the minus from the consideration on the easiness of dielectric breakdown in all the conventional methods.
FIG. 3 shows a situation where a ball production is conducted against a copper wire having a naturally oxidated film of about 10 .ANG. at the surface of the copper wire by a method similar to that which is executed to the conventional gold wire. In FIG. 3, the reference numeral 1 designates a copper wire, the reference numeral 2 designates a discharge electrode, the reference numeral 3 designates an arc, the reference numeral 4 designates a power supply, the reference numeral 5 designates a capillary chip as a bonding tool, and the reference numeral 6 designates a portion at which an electrode is produced, and to which heat is given. The reference numeral 7 designates an inactive gas ambient such as argon, and the reference numeral 8 designates a portion produced with receiving the melted hysteresis.
In this method there is a physical scientific nature that the hot electron emission is willing to broaden the electrode region by searching a more stable point where the work function is small (that is, quite a thin metal oxide film) at the side of the minus electrode at the discharging. Accordingly, in a case where the wire 1 is at a minus voltage an arc 3 is produced up to a portion above the wire 1 as shown in FIG. 3(a), and heat is given from the surface portion of the wire 1. As a result, it is likely to generate a faulty ball 8 with remaining a core, that is, an unmelted portion 1a of the wire 1 at the top end thereof. This faulty ball 8 remains to include an unmelted portion 1a inside thereof although it has a spherical configuration, and the ball including the unmelted portion is difficult to be transformed when it is bonded to the aluminum electrode on a silicon chip. Such a property of the prior art ball may cause damages to arise to the silicon chip.
However, in a case of using a gold wire in a conventional method there arises no such problem because the gold wire produces no oxides at the surface thereof. Then, the wire is placed at the minus side as described above from the consideration on the easiness of occurring the discharge.